[edit: for clarity, I am not against roundabouts]

I don't have real expertise in civil engineering

Traffic lights have a smaller physical footprint and require less construction activity.

Traffic lights fit on a cost spectrum from two stop sighs, to four stop signs, to stop signs plus blinking lights, to non-trivial signaling.

First make it work. Then make it good. Then make it fast.

Engineering is not a one size fits all discipline.

[edit: I have a background in urban planning, architecture, and real-estate development]